1. Field of the Invention
This invention is related to the field of radio frequency (RF) signal processing, specifically RF mixers.
2. Related Art
Conventional switching mixers used in RF transceivers generate harmonics of a local oscillator (“LO”) frequency. Two main contributors of these harmonics are the shape of the LO waveform and the switching action of the mixer. The LO waveform shape usually resembles a square wave more than a sinusoidal wave, especially at low LO frequencies. A square wave, unlike a single-tone sinusoidal wave, contains harmonics at odd multiples of the main frequency. These harmonics appear at the mixer output when a square wave is used as a LO signal. Further, even if a sinusoidal LO signal were employed, harmonics would still exist because of the inherent switching action of LO transistor pairs in the mixer.
In a typical transmitter where up-conversion is required, these harmonics create copies of a baseband signal at odd multiples of the LO frequency. To attenuate these unwanted harmonics, a post-filtering is required. This post-filtering adds noise, power, and complexity to the system.
Mixers used in receiver applications are also affected by LO harmonics. For example, in a direct down-conversion receiver, RF input signals at odd multiples of the LO frequency land directly on top of each other at the mixer output. In this case, a pre-filtering is required to attenuate the signals near the frequency of the odd LO harmonics at the input, prior to the mixer stage.
Therefore, what is needed is a mixer that does not produce strong mixing products with harmonics of the LO signal, yet reduces noise and decreases sensitivity to rise and fall times of an input signal.